1. Field of the Invention
This invention generally relates to voltage-variable capacitors (varactors) of a parallel plate design.
2. Description of the Related Art
Capacitors are a basic building block for electronic circuits and voltage-variable capacitors (varactors) have the added flexibility that their capacitance can be tuned by changing a bias voltage across the capacitor. Dielectric materials which have a permittivity that depends on the applied electric field can be used to form such varactors. Varactors have an added advantage that they can be easily integrated with other components, particularly if the dielectric layer is a thin film. One common approach to voltage-variable varactors is the xe2x80x9cparallel-platexe2x80x9d configuration, in which the voltage-variable dielectric is sandwiched between two electrodes. For example, in an integrated varactor, one electrode may be a bottom conducting layer, the dielectric may be a ferroelectric thin film deposited over the bottom electrode, and the top electrode may be a metal layer.
In the parallel-plate configuration, the capacitance of the varactor is determined in part by the area of overlap of the top electrode, the dielectric layer and bottom electrode. For convenience, this area shall be referred to as the active region of the varactor. In many designs, the active region is determined mainly by the size and shape of the two electrodes; the dielectric layer is made large enough so that it does not additionally limit the active region. Thus, the varactor is designed for a specific capacitance by adjusting the lateral dimensions of the top and/or bottom electrodes. The active region typically is square-shaped (or close to square-shaped) although other shapes, including circular, may also be used.
The electrodes have some finite resistance. This resistance leads to loss and also limits the operating bandwidth of the varactor. For example, the electrode resistance in series with the varactor""s capacitance forms an RC combination with a certain time constant. Higher resistance means longer RC time constant and lower cutoff frequency. The resistance typically is reduced by increasing the thickness of the metal films forming the electrodes. However, limitations in the fabrication process can place an upper limit on the maximum thickness of the electrodes. Increasing the thickness of the electrodes can also be costly since, for various reasons, the bottom electrode may be made from an expensive refractory metal such as platinum, palladium, iridium and related compounds. For these reasons, the electrode thicknesses are constrained. This, in turn, limits the sheet resistance and the current handling capacity of the varactor as a result of effects such as electromigration and/or Joule-heating. Hence, the conventional parallel-plate design is not particularly well suited for implementing low-loss, high-current varactors.
The present invention overcomes the limitations of the related art by providing a parallel-plate varactor in which the current conducting perimeter of the active region for at least one electrode is increased relative to the area of the active region. The current conducting perimeter is that portion of the geometrical perimeter which supports current flow between the active region and the rest of the electrode. In one approach, the current conducting perimeter is increased by changing the shape of the active region, for example by using a long skinny active region rather than a square one. In another approach, the active region is implemented by a number of disjoint subregions, termed xe2x80x9ccells,xe2x80x9d which are coupled in parallel. The cells together have the area required to implement a certain capacitance but subdividing the active region into cells increases the total current conducting perimeter.
Increasing the current conducting perimeter addresses the problems with conventional parallel-plate designs. The increased perimeter results in more paths for current to move between the dielectric layer and the bulk regions of the electrodes, thus reducing the resistance of the electrodes. This same effect also increases the current handling capacity of the varactor. Furthermore, these gains are achieved without having to increase the electrode thickness, although doing so may result in even further gains.
In one implementation, a parallel plate varactor includes a bottom electrode, a top electrode and a dielectric layer sandwiched between the top electrode and the bottom electrode. The permittivity of the dielectric layer varies according to an electric field applied to the dielectric layer. The active region of the varactor is defined by an overlap between the top electrode, the dielectric layer and the bottom electrode. For at least one of the electrodes, the resistance of the active region of the electrode is significantly higher than a resistance of a bulk region of the electrode. Furthermore, the active region has a lateral area A, the electrode has a current conducting perimeter P, and a ratio R of the perimeter P to a square root of the area A is at least 2.0.
In one embodiment, the dielectric layer is a voltage-variable thin film (e.g., based on a ferroelectric material) and the high resistance electrode is a refractory metal. Examples of voltage-variable ferroelectric thin films include barium titanate, strontium titanate and barium strontium titanate. Examples of refractory metals include platinum, palladium, iridium, nickel, tungsten, or ruthenium.
In one particular aspect of the invention, the active region includes one or more rectangular cells. The active region of the bottom electrode has a higher sheet resistance than the active region of the top electrode. For example, platinum may be used for the bottom electrode, barium strontium titanate as the dielectric layer and gold for the top electrode. Furthermore, for each cell, the current conducting perimeter of the bottom electrode includes at least three sides of the cell and the current conducting perimeter of the top electrode includes the fourth side of the cell.